Method of manufacture of articles from metal powders



Aug. 7, 1956 A. L. BOEGEHOLD ET AL 2,7

METHOD OF MANUFACTURE OF ARTICLES FROM METAL POWDERS Filed June 4, 1952 MIX POh/DfFt smoumz JINTER H07 FOR 65 f/NISH m/aw A. 50:60am? 5904 J 5MP:

flr/m/y 5 7054/) INVENTOR5 iatented Aug. 7, 1 956 METHOD OF MANUFACTURE OF ARTICLES FROM METAL POWDERS Alfred L. Boegehold, Detroit, Mich, and Paul J. Shipe and Athan Stosuy, Dayton, Ohio, assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware Application June 4, 1952, Serial No. 291,752 4 Claims. (Cl. zap-420.5)

This invention is directed to the critical manufacture of blades for use in turbo-compressors and the like wherein said blades are made from metal powder.

In the manufacture of turbo-compressor blades, high strength, ductility, and extreme accuracy are two factors.

The dimensions of the blades, which must be held to within extremely close tolerances, are diflicult to maintain by conventional manufacturing procedures. The blade has a cupped contour and a twist therein which is difiicult to form in normal stamping and machining practice.-

Blades of this character are therefore ideally suited for manufacture from metal powders wherein exact contours and twists may be maintained through the briquetting of the metal powder into the desired shape and subsequent sizing operations.

Articles made from metal powders by usual sintering procedures, however, lack the strength and corrosion resistance required in turbo-compressor blades. This is explained by the fact that such articles have a degree of porosity after briquetting and sintering, which porosity presents an activated surface, due to the large area thereof, toward corrosive action and simultaneously due to the fact that porous metal structure has reduced physical properties compared to solid metal.

For these reasons, it was necessary to develop an essentially new material together with the method of making such material whereby powder metallurgy could be adapted for use in precision manufacture of turbo-compressor blades having physical properties and corrosion resistance comparable to blades made from solid metal, which blades, however, are capable of being made at reduced costs over blades made from solid metal due to the fact that the briquetting of the blade can be actually controlled to yield exact contours and twists.

We are aware of course of porous metal blades for use in turbo-compressors wherein the porosity of the blade has been destroyed through infiltration of a lower melting point metal, such as copper or cuprous alloys, to fill the voids of the porous structure and thereby add additional strength to the blade. Such blades are described in copending application Serial No. 290,122, filed May 27, 1952.

The present invention is directed to the precision manufacture of turbo-compressor blades and the like wherein no infiltrated metal is used and wherein the physical properties and corrosion resistance of the blades are fully comparable to blades made by any other process heretofore known.

In order to accomplish these ends, it is necessary to hot press, or hot forge, the sintered briquet to bring it to a more dense condition whereupon the physical properties are increased tremendously, the corrosive tendency is reduced to a minimum and wherein the dimensions of the blade are maintained within precision limits. While thejmere hot pressing of sintered metal briquets is not new, .we have found that there are a number of control I factors which are imperative to follow if a satisfactory blade is to be produced, which are novel and which open a new field of operations in powder metallurgy.

Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawing, wherein preferred embodiments of the present invention are clearly shown.

In the drawing:

Fig. 1 is a flow chart of the steps of the process.

Fig. 2 is a view in perspective of a representative type of compressor blade.

Specifically, the process is as follows: Iron powder, preferably of about 100 mesh size, is mixed with graphite. We prefer, in this instance, to use reduced oxide iron powder due to the improved strength in the green briquets. This iron powder is mixed with about 2.25% graphite to normally obtain an .85% combined carbon content in the finished sintered briquet. The reduction in the quantity of carbon is explained by the fact that the iron powder usually includes small quantities of iron oxide whichare reduced by some of the graphite whereby the difference between the starting percentage of 2.25 and the finished percentage .85 carbon is taken up in the oxide reduction process. It is apparent here that if the iron powder includes greater quantities of oxide the initial graphite addition must be increased whereas if the iron powder includes lesser quantities of iron oxide, the graphite addition may be reduced. In all cases, this addition should be controlled by a test run in order to determine the exact amount of carbon in the finished article.

The mixture of iron powder and graphite is next briquetted to the desired shape and contour. This is done in conventional briquetting dies at about 60,000 pounds per square inch pressure. At this point, it is to be understood that greaterdensity can be obtained if the briquetting pressure is increased. However, die-wear, stripping pressure and other manufacturing problems are directly proportional to the briquetting pressure and therefore .we propose to use as low a briquetting pressure as is compatible with the necessary density required for the subsequent operations. In this respect .we :know that theoretical density of iron is in the order of 129 grams per cubic inch. We have discovered that the sintered briquet to be useful .in our process must have a density equal to 97 grams per cubic inch, minimum, or about of the theoretical density. This figure is important and is based on the fact that in the subsequent heating operation for forging, if the density is less than this figure, excessive internal oxidation occurs which ruins the strength .of the finished part. Therefore, it is of utmost importance that the minimum density of the vbriquet be maintained at 75% minimum, of the theoretical density of the material being used and for this purpose with reduced oxide iron powder of mesh, we have found that 60,000 pounds per square inch is the desired briquetting pressure when based on the necessary physical characteristics of the briquet as compared to diewear, stripping pressures and the like. I

The formed briquets are next sintered in the conventional type furnace at about 2050 F. for about fortyfive minutes in an atmosphere which is non-oxidizing to the material and which does not have any decarburizing tendencies. incompletely burned natural gas is one of the most suitable atmospheres due to its low cost and availability although any of the usual non-oxidizing or reducing sintering atmospheres may be used if they meet the specifications above. The sintered briquets are next cooled in the same atmosphere and are then set aside for the subsequent forging operation.

This is accomplished at the same temperature as used in sintering and in this respect the sintered articles may be reheated in a furnace having a suitable non-oxidizing atmosphere or they may be reheated in air by electrical induction heating. No excessive oxidation takes place here since the rate of heating is. so rapid that little effect is noticed from the atmosphere. The hot parts at 2050 F. are then transferred to a forging die. This transfer takes place in the order of from one tofour seconds according to the speed of the operator or mechanism and we have found that with a-controlled density of the part as noted above, 75% of the theoretical density minimum, that no excessive oxidation takes place during the travel of the part from the reheating equipment to the forging die. It is apparent that a hood or tunnel may be provided to include an atmosphere therein between the various pieces of equipment but our experience has shown this to be unnecessary if the density of the briquet is properly controlled. The hot part is then forged at about 100 tons per square inch to provide a precise contour and a desired density.

In the forging operation, the density of the part is increased to about a minimum of 123 grams per cubic inch as compared with 129 grams per cubic inch as a theoretical figure. In other words, the density is increased to a minimum of about 95% of theoretical density. The part may then be cooled slowly in the atmosphere without any deleterious oxidation or it may be cooled more rapidly in a suitable coolant. In the forging step, the blade is formed to close tolerances and the twist may be imparted thereto.

The next step involved is that of finishing. This may include several operations, for example, the flash as oocasioned by the forging step may be trimmed off and the entire article may be ground to precision lim-its. In this instance, it is pointed out that since the article is briqueted to a closely controlled size, prior to hot forging to precision limits, that very little material is removed during the grinding step which is best done with a contour grinder or some other automatic equipment for removing metal. Since it is possible to briquette and forge a part to extremely close tolerances, it is conceivable that in many applications, no further finishing operations are required, the accuracy requirements of the part or surface finish required more or less dictating the finishing operations necessary. Also heat treating steps may follow forging and these conventional operations are therefore included in the term finishing.

Several considerations are of great importance in the forging step. These concern the uniformity in density in the finished part and are determined by the direction of compression, amount of compression in terms of material, and internal flow of particles during the forging operation. We have found that the forging must all take place in one direction and theoretically at least, if the part and the forging die could be so dimensioned that there is no clearance in a direction opposite to the direction of compression, best results would be obtained. However, such a fit is commercially impossible and therefore a minimum clearance is provided to permit easy positioning of the part in the die. The material being of a porous nature is largely compressible within itself and therefore it is briquetted to a shape wherein every dimension of thickness in the direction of the forging operation is onethird greater than that ultimately desired. This produces the entire change of shape in the article in one direction of movement as is permitted mainly by the compressibility of the material. We have found that the density of the finished part is never uniform if overall thickness of every dimension in the direction of forge is not maintained within the same ratio. This movement in one direction has still another function. It causes an internal fiow of particles during the forging operation which improves the strength and knits the article together into a stronger framework. We have found that it is necessary to have approximately 5% internal flow of particles during forging if the physical properties of the part are to be maintained above minimum limits. This flow is occasioned by the various thicknesses of the part, it being apparent that these varying thicknesses cause movement in one section of the article followed by another and another, etc. The flow therefore produces considerable strength at the joining line between portions of the article of different thicknesses, etc. We have found that a hot forged and heat treated blade of a theoretical density has physical properties approximating conventional wrought materials of similar composition.

It is apparent that the blade after the finishing and heat treatment operations may be treated in any desired manner, such as by electroplating, atmospheric treatments and the like to obtain a desired surface and finish.

The procedure of forming a blade by powder metallurgical procedures as hereinbefore noted provides a precision method for accomplishing a given end. It permits accurate contouring of the blade surface in every section thereof since once the briqueting and forging dies are completed, every blade made therein is identical, thereby eliminating differences in twist and contour as occasioned by conventional manufacture wherein physical properties of the particular metal being operated upon cause variations in dimension.

While the embodiments of the present invention as herein disclosed, constitute preferred forms, it is to be understood that other forms might be adopted.

What is claimed is as follows:

1. The steps in a method of forming high strength articles from metal powders, comprising, mixing iron and graphite powders in desired proportions, briquetting the powder to the desired shape and contour wherein the density of the briquetted article is approximately 7 5% of the theoretical density of a wrought material of a similar composition, sintering the briquet so formed under suitable conditions of time, temperature and atmosphere for forming a strong porous metal article including carbon in the combined form, and then hot forging the article to a minimum density of 95 of the theoretical density wherein the entire change of shape of the article takes place in one direction of movement and wherein the minimum internal flow of the particles within the article is at least 5%, and finally finishing the forged article.

2. The steps in a method of forming high strength articles from metal powders, comprising, mixing iron and graphite powders in desired proportions, briquetting the powder to the desired shape and contour wherein the density of the briquetted article is approximately 75% of the theoretical density of a wrought material of a similar composition, sintering the briquet so formed under suitable conditions of time, temperature and atmosphere for forming a strong porous metal article including carbon in the combined form, and then hot forging the article to a minimum density of 95 of the theoretical density wherein the entire change of shape of the article takes place in one direction of movement and wherein the minimum internal flow of the particles within the article is at least 5%, and finally heat treating the hot forged article. i i t 3. The steps in the method of forming compressor blades for use in axial flow compressors and the like directly from metal powders comprising, the steps of mixing the iron and graphite powders in the desired proportions so that the finished article includes combined carbon in the neighborhood of .85 ,briquetting the mixture of iron and graphite powder to the desired shape and contour and to a density of approximately 75% of the theoretical density of a wrought material of a similar composition, sintering the briquet so formed at a temperature in the order of 2000 F. to 2050 F. for a time sulficient to cause carbon diffusion in a controlled atmosphere and then hot forging the article to the exact shape desired and at a temperature of around 2000 F. to 2050 F. to a minimum density of about 95% of the theoretical density and ma direct ion whereby the entire change of shape of the article takes place in one direction of movement with a minimum internal flow of particles within the article of at least 5% and finally finishing the forged blade.

4. The steps in the method of forming compressor blades for use in axial flow compressors and the like directly from metal powders comprising, the steps of mixing the iron and graphite powders in the desired proportions so that the finished article includes combined carbon in the neighborhood of .85 briquetting the mixture of iron and graphite powder to the desired shape and contour and to a density of approximately 75% of the theoretical density of a Wrought material of a similar composition, sintering the briquet so formed at a temperature in the order of 2000 F. to 2050 F. for a time sufficient to cause carbon difiusion in a controlled atmosphere and then hot forging the article to the exact shape desired and at a temperature of around 2000 F. to 2050 F. to a minimum density of about 95% of the theoretical density and in a direction whereby the entire change of shape of the article takes place in one direction of movement with a minimum internal flow of particles within the article of at least 5% and then heat treating and finishing the forged blade.

References Cited in the file of this patent UNITED STATES PATENTS 

1. THE STEPS IN A METHOD OF FORMING HIGH STRENGTH ARTICLES FROM METAL POWDERS, COMPRISING, MIXING IRON AND GRAPHIC POWDERS IN DESIRED PROPORTIONS, BRIQUETTING THE POWER TO THE DESIRED SHAPE AND CONTOUR WHEREIN THE DENSITY OF THE BRIQUETTED ARTICLE IS APPROXIMATELY 75% OF THE THEORETICAL DENSITY OF A WROUGHT MATERIAL OF A SIMILAR COMPOSITION, SINTERING THE BRIQUET SO FORMED UNDER SUITABLE CONDITIONS OF TIME, TEMPERATURE AND ATMOSPHERE FOR FORMING A STRONG POROUS METAL ARTICLE INCLUDING CARBON IN THE COMBINED FORM, AND THEN HOT FORGING THE ARTICLE TO A MINIMUM DENSITY OF 95% OF THE THEORETICAL DENSITY WHEREIN THE ENTIRE CHANGE OF SHAPE OF THE ARTICLE TAKES PALCE IN ONE DIRECTION OF MOVEMENT AND WHEREIN THE MINIMUM INTERNAL FLOW OF THE PARTICLES WITHIN THE ARTICLE IS AT LEAST 5%, AND FINALLY FINISHING THE FORGED ARTICLE. 